248 AMERICAN TESTUDINATA. Part II. 



this second sub-order, called Amydaj by Oppel. In the first place, we notice 

 the Trionychidaj, so remarkable for the peculiarities already alluded to ; next we 

 have the North American Chelydroidae with their fossil European representative ; next 

 the South American Chelyoidae, the HydraspididaB, the Cinosternoidae, the Emydoidaj 

 proper; and lastly, the Testudinina, each of which groups presents typical patterns 

 of form which are constant within their limits, and strikingly contrasted when 

 compared with one another. For it is not true, as is so frequently repeated, that 

 the fresh-water Turtles are flat and broad when compared with the land Turtles. 

 Some of our marsh Turtles, and especially our Ozotheca, are quite as high compara- 

 tively, and certainly as narrow as any of the land Turtles, whilst the Chelydroidae 

 with their carinated backs, their dentated margin, their broad, flat heads, their 

 narrow, cross-like sternum, their large tail, their imperfectly retractile limbs and 

 head, differ far more from the other Emydoida? than any land Turtles. I do 

 not, therefore, hesitate for a moment to consider these two groups as two distinct 

 families. Of the family of ChelydroidiB, there are two species in the United States 

 belonging to two distinct genera, as I have ascertained that Chelydra Serpentina 

 differs generically from the Chel. Temminckii And., for which I have proposed the 

 name of Gypochchjs TemmincMi. Their thoroughly aquatic haljits show them to be, 

 next to Trionyx and Chelys, the lowest family among Amydte. Next to them, 

 I would place the family of Cinosternoids, on account of their less extensive sternum 

 and of their more movable pelvis. Tliere can be no doulit that they constitute 

 a family by themselves, when in addition to the difference of form already alluded 

 to it is found that they have no odd bone in the sternum, so that their lower 

 shield divides into symmetrical halves, along an uninterrupted straight suture, fol- 

 lowing the middle line. The long-necked Hydraspids with retractile head, or rather 

 whose head can be bent laterally and so protected under the shield, come next 

 in order; but as they are all foreign to the United States, and I have had few 

 opportunities for their study, I must omit any further mention of them. I would 

 only recall, in this connection, the interesting fact that the types of land and 

 fresh-water Turtles are so localized upon the surface of the globe, that, though the 

 number of Testudinata is very great in the United States, not a single Hydraspid, 

 for instance, is found within their limits, and only two Testudos occur in theii' 

 southern parts, while the family of Chelydi'oids, on the contrary, belongs almost 

 exclusively here, and is only found again in China. The true home of the 

 genuine Emydoids is alsa North America, as the true home of the Chelyoids 

 and Hydraspids is South America, though a few species of the latter family occur 

 also in other parts of the world. 



As a family, the Emydoidae are easily characterized by their ovate form, swelling 

 centrally, wliile the margm has a tendency to spread outward, in which last featiu-e 



